Lambda probes are used in the exhaust branch of internal combustion engines, for example, for measuring the oxygen concentration of the exhaust gas, in order to control the preparation of the fuel-air mixture of the internal combustion engine. They utilize the property of a solid electrolyte composed of a zirconium dioxide ceramic, which, above a temperature of approximately 300° C., is permeable to oxygen ions. Platinum electrodes are mounted on at least two sides of the solid electrolyte. If one of the sides is exposed to an exhaust gas having decreased partial oxygen pressure, and the other side to a reference gas, for example air, oxygen ions diffuse through the solid electrolyte. The resulting potential difference between the two electrodes of such a concentration cell is described by the Nernst equation and can be used to determine the lambda value of the exhaust gas within a narrow window around lambda=1. The lambda value represents the existing air-fuel ratio relative to a stoichiometric air-fuel ratio.
Oxygen ions can be pumped through the solid electrolyte in response to application of an external voltage to the electrodes. This property is utilized in what are commonly known as broadband lambda probes to determine the lambda value within a broad range from lean to rich exhaust gas. To this end, an electrode, the external pump electrode, faces the exhaust gas. As the inner pump electrode, the second electrode communicates via a diffusion barrier with the exhaust gas. The second electrode can be configured in a measuring cell in the solid electrolyte that communicates with the exhaust gas through a diffusion channel and the diffusion barrier. In one alternative configuration, both electrodes are disposed on the side of the solid electrolyte facing the exhaust gas, the inner pump electrode being covered by an applied diffusion barrier layer.
The outer pump electrode, the inner pump electrode and the solid electrolyte disposed therebetween form what it commonly known as a pump cell. In response to the application of a voltage, oxygen ions are transported from the inner pump electrode to the outer pump electrode. If the voltage is high enough, a limiting current is reached that is determined by the oxygen diffusion through the diffusion barrier. The oxygen diffusion, and thus the limiting current measured to determine the lambda value, are directly dependent on the partial oxygen pressure in the exhaust gas, as well as on the diffusion properties of the diffusion barrier. The known diffusion properties of the diffusion barrier make it possible for the lambda of the exhaust gas to be determined from the limiting current.
Another structure known as a two-cell broadband lambda probe is formed from a combination of a pump cell and a concentration cell. The outer pump electrode faces the exhaust gas, and the inner pump electrode is configured in a measuring cell that communicates with the exhaust gas through the diffusion barrier. In addition, a first electrode of the concentration cell, referred to as measuring cell, is located in the measuring cell. As a second electrode of the concentration cell, a reference electrode is mounted on the solid electrolyte in a separate reference channel. The reference channel is filled through an external opening with a reference gas having a defined oxygen concentration, preferably with air.
In the case of such a two-cell broadband lambda probe, the pump cell pumps oxygen ions in or out to adjust the lambda value in the measuring cell to a lambda of preferably 1. To this end, the lambda value in the measuring cell is measured via the concentration cell and, by appropriately regulating the pumping current, regulated by the pump cell to λ=1. The pumping current required for that purpose is dependent on the oxygen quantity diffusing through the diffusion barrier into the measuring cell, and thus on the lambda of the exhaust gas, and the diffusion properties of the diffusion barrier. When the properties of the diffusion barrier are known, the lambda of the exhaust gas can be determined from the pumping current.
The diffusion properties of the diffusion barriers are subject to a substantial manufacturing variance. Therefore, the individual diffusion properties must be taken into account during analysis of the broadband lambda probe signals.
German Published Patent Application No. 10 2008 002 734 A1 describes an example of a broadband lambda probe.
German Published Patent Application No. 10 2010 000 663 A1, for example, discusses devices used as evaluation and control units for operating a broadband lambda probe. The fundamental idea, the structure and the basic function of the control unit described in this publication essentially correspond to the evaluation and control unit discussed in German Published Patent Application No. 10 2008 001 697 A1 of the Applicant. The control unit is designed as an ASIC and is known by the name CJ 125 or CJ 135 (see Product Information of the Applicant entitled “CJ135—Lambda Probe Interface IC”).
German Published Patent Application No. 10 2010 039 188 A1 describes a method for sensing at least one property of a gas in a measuring cell, in particular for identifying a component of the gas. The method provides for using at least one sensor element having at least one cell. The cell has at least one first electrode, at least one second electrode, and at least one solid electrolyte through which the first electrode and the second electrode communicate with one another. The first electrode is able to receive gas from the measuring cell. The second electrode communicates with at least one reference gas cell that is adapted for storing a volume of a component of the gas. The method encompasses at least two operating modes:                at least one measurement mode; in the measurement mode, the cell being operated as a pump cell, and the property being inferred from at least one pumping current through the pump cell;        and at least one diagnostic mode, a storage capacity of the reference gas cell being examined in the diagnostic mode, a measured quantity influenced by at least one Nernst potential applied to the cell being recorded, and the storage capacity being inferred from the measured quantity.        
For the operating principle thereof, a good portion of the lambda probes that are presently commercially available requires what is commonly known as an air reference (actually oxygen reference), that has the characteristic that the oxygen concentration of this air reference is 21% oxygen or higher. Certain inherent risks are associated with all of the methods that utilize this air reference since it does not contain enough oxygen, respectively is “contaminated” by elements where oxygen has a negative valence. This erroneous reference results in a displacement of the lambda characteristic curve and, as a logical consequence, in a faulty lambda determination, and sometimes in unwanted error entries in the software of the engine control unit. It is usually not possible to diagnose the air reference in conventional evaluation circuits, and allowances must be made for the residual risk described above.